The field of photomechanical process, demands photographic materials that give excellent reproducibility of originals, are processable with stable processing solutions that are simple to replenish, and so on in order to cope with the diversity and complexity of printed matter.
In particular, a line original used in the photograph-taking process is made by putting together photocomposed letters, handwritten letters, illustrations, halftone photographs, and so on, so it has a mixture of images differing in density and line width from one another. Under such a situation, it has been strongly desired to develop such process cameras, photographic light-sensitive materials and image forming methods as to duplicate line originals with good reproducibility.
In the photomechanical process for catalogues and large-sized posters, on the other hand, magnification (spread) or reduction (choke) of halftone photographs is normally carried out. In the case of magnification, lines are sparsely present in the photomechanical process using expanded dots, and photographs of blurred dots are taken. In the case of the reduction, the number of lines per inch becomes greater than those of the originals, so halftone photographs of the smaller dot areas are taken. Accordingly, image forming methods which can ensure much wider latitude than conventional ones have been required for retaining the reproducibility of screen range.
As for the light source of a process camera, a halogen lamp or a xenon lamp is used. For the purpose of gaining photograph-taking sensitivity of these light sources, photographic light-sensitive materials are generally subjected to orthochromatic sensitization. However, it has turned out that orthochromatically sensitized photographic materials are more strongly influenced by the chromatic aberration of the lens used, which results in a deterioration in quality of the images formed. This kind of deterioration is more conspicuous when a xenon lamp is used as light source.
A system that has been known to meet the demand for wide latitude, uses a lithographic silver halide photosensitive material comprising silver chlorobromide (having a chloride content of at least 50%) processed with a hydroquinone developer in which the effective concentration of sulfite ion is extremely low. This results in a line or dot image with high enough contrast and high enough photographic density to clearly distinguish the image area from the non-image area. In this system, however, the developer used is quite sensitive to air oxidation because of a low sulfite ion concentration. Various efforts and contrivances have been made to maintain the developer activity constant. In the present situation, some of them, though practically used, have a very slow processing speed which results in the lowering of working efficiency.
Thus, there is a need for image forming systems which can resolve the instability of image formation in the above-described developing method (lithographic developing system) by using a processing solution with a high storage stability upon development and can provide very high contrast photographic characteristics. One such system has been proposed using silver halide photographic material having high surface sensitivity, but low internal sensitivity, and containing a specified acylhydrazine compound as an additive. This material is processed with a developer which contains a sulfite preservative in a concentration of at least 0.15 mol/l and is adjusted to pH 11.0 to 12.3 to produce a very high contrast negative image with a gamma value greater than 10. This material is disclosed in U.S. Pat. Nos. 4,166,742; 4,168,977; 4,221,857; 4,224,401; 4,243,739; 4,272,606; and 4,311,781. This image-forming system can use silver iodobromide and silver chloroiodobromide in addition to silver chlorobromide. This is in contrast to the conventional system for forming a very high contrast image where only silver chlorobromide with a high chloride content is used.
The foregoing image forming system has excellent properties with respect to sharpness, quality of halftone image, stability and rapidity of processing, and reproducibility of the original. But systems that yield further improvement of the reproducibility of an original are desired in order to cope with the up-to-date diversity of printed matter.
For gathering and contact works improvement has been directed to increasing work efficiency by developing material that can be used in a better-lighted environment than previously known. This aim has led to the development of photosensitive materials and exposure printers for graphic arts which can be handled in an environment that is substantially "daylight".
The term daylight photosensitive material as used herein describes a photosensitive material of the kind which can be handled safely for a long period of time using a safe-light with rays not including the ultraviolet wavelengths of 400 nm or longer.
Daylight photosensitive material to be employed in gathering and contact works is utilized for effecting negative-positive conversion or positive-positive conversion by using as originals development-processed films having letter or halftone images, and subjecting the originals and a contact photosensitive material to contact exposure. In addition, it has been required that this daylight photosensitive material have (1) properties making it feasible for halftone, line and letter images to undergo negative image-positive image conversion in accordance with individual dot areas, line widths and letter image widths, respectively, and (2) properties permitting the tone control of halftone images, and the line width control of line and letter images. Daylight contact photosensitive materials capable of meeting these requirements have been available.
However, in high level image-conversion work for forming white on-black letter images by superimposition contact work, the conventional method of using a daylight photosensitive material and carrying out the contact work in daylight was defective. This conventional method gave white-on-black letter images inferior in quality to those provided by using a conventional dark-room contact photosensitive material and carrying out the contact work in dark room.
The method of forming white-on black letter image through the superimposition contact work is described in more detail below.
As shown in FIG. 1 hereinafter, a letter or line image-formed film (line original) (b) stuck to a transparent or translucent base (a); and a halftone image-formed film (halftone original) (d) stuck to a transparent or translucent base (c), (wherein a polyethylene terephthalate film having a thickness of about 100 microns is generally used as the sticking base) are superposed, and employed as an original. The emulsion surface of a contact photosensitive material (e) is brought into direct contact with the halftone original (d), and optically exposed.
After exposure, the contact photosensitive material is development-processed to produce white areas corresponding to line images inside the black halftone images.
A point of importance in the above-described method for forming white-on-black letter images is that the ideal of negative image/positive image conversion consists in accomplishing the conversion in accordance with individual dot areas of a halftone original and individual line widths of a line original, respectively. However, as is apparent from FIG. 1, the exposure for printing the line original (b) on the contact photosensitive material (e) is carried out with the sticking base (c) and the halftone original (d) sandwiched in between. This is in contrast to the exposure carried out for the halftone original (d) where the halftone original (d) is in direct contact with the emulsion surface of the contact photosensitive material.
Therefore, the optimum exposure for faithful negative image/positive image conversion with respect to the halftone original results in an out of focus line original because the sticking base (c) and the halftone image (d) are interposed as a spacer. As the result, narrowing of the line width of white-printed image corresponding to the line original is caused. This is responsible for deterioration in quality of the white-on-black letter image.
With the intention of resolving this problem, systems using a hydrazine compound are disclosed in JP-A-62-80640 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-62-235938, JP-A-62-235909, JP-A-63-104046, JP-A-63-103235, JP-A-63-296031, JP-A-63-314541, and JP-A-64-13545. However these systems are not satisfactory, so it is desired that further improvements be introduced that overcome the above described problems.
As an attempt for improving image quality, there has also been known a method of releasing a development inhibitor from a redox compound containing a carbonyl group, in a distribution that corresponds to the silver image. This is disclosed, e.g., in JP-A-61-213847. However, this method has defects. Since the extension of screen range is insufficient and the range of image-tone control is narrower than that in a lithographic development system, notwithstanding the use of the redox compound, the method cannot function as a contrast development system for photographing a halftone image. Further, as the nucleation activity fluctuates from too high to too low depending on fluctuation in the developer composition (e.g., pH, concentration of sodium sulfite, etc.), the images obtained lack uniformity in quality and their value as commodities is impaired.
Therefore, photosensitive materials which enable the formation of high contrast halftone images using a stable developer and having controlled image tone over a wide range are desired.